A Quartz Crystal Microbalance (QCM) sensor arrangement utilizes the piezoelectric effect of a quartz crystal. In such a system a quartz crystal that is placed between two electrodes, which are connected to an AC-potential, begins to oscillate if the frequency of the AC-potential is close to the resonance frequency of the oscillation mode for the quartz crystal.
A typical QCM sensor arrangement comprises a thickness shear mode piezoelectric sensor unit, a sample insertion unit, a frequency counter, and signal presentation equipment and buffer and waste containers. A sample, which can contain any chemical substance of interest that can interact with the electrode of the sensor, is introduced into the sensor unit by the sample insertion unit. The sensor unit contains a piezoelectric resonator, a sensor chamber, flow channels to and from the chamber and an oscillating circuit. The sample induces an interaction with the piezoelectric sensor surface, which can in turn be observed by monitoring the oscillating characteristics of the crystal plate, e.g. by measuring changes in the piezoelectric resonator frequency, which resonator frequency typically is 5-50 MHz. The resonator utilized by the sensor unit includes a crystal plate, which is provided with electric contact areas for an electrode and a counter electrode on its surface, which electrodes are connectable to a signal source (e.g. an alternating voltage source) as well as to a measurement device. For measuring, one side of the piezoelectric crystal plate is brought into contact with the sample to be examined.
The crystal responds to the substance to be detected or to a change in the physical properties of the sample by an increase of associated mass on the electrode surface or a loss of associated mass on the electrode (i.e. a change in the mass that oscillates together with the crystal). Such a change in associated mass leads to an alteration of the resonance frequency and/or oscillation amplitude of the resonator.
Piezoelectric sensors can be used for analysis of the viscosity of a liquid sample and are particularly suitable for studying chemical and biochemical interactions.
The crystal plates utilised by piezoelectric resonators are usually oscillator quartz crystal plates. “A Novel Monolithic Piezoelectric Sensor”, Schweyer et. al., 1997 IEEE International Frequency Control Symposium discloses a piezoelectric sensor which may be used for measuring purposes. In QCM sensor arrangements the resonator is typically arranged in a sensing chamber in which the sample is brought into contact with one of the electrodes (sensing electrode) of the resonator. Such a sensor arrangement is disclosed in EP 0 453 820 A2. The sensing chamber is often designed as a flow-through-cell, (see e.g. U.S. Pat. No. 6,196,059) such that the sample flow has its inlet at one end of the chamber and its outlet at another, whereby the sample can be lead though the chamber and meanwhile interact with the sensing electrode during its passage through the chamber. There is a continuously increasing demand of QCM sensor arrangements that have an improved sensitivity.
The object of the present invention is to provide a resonator and a flow cell for use in a sensor arrangement that has an improved sensitivity.